The overall goal of this project is to determine how sickle cell disease may alter the characteristics of the hemopoietic stem cell population and the way how hematopoiesis responds to the chronic hemolysis. Over the past year our attention was focused on the Identification of hemopoietic Growth Factors specifically associated with SS anemia and on evaluation of the level of erythropoiesis in relation to the peripheral HbF levels in SS patients. Our earlier reports demonstrated that more severe forms of SS anemia (LFSS patients) are associated with increased levels of plasma GM-CSF, but not IL-3, and lack of BFU-E inhibitory activity in monocyte conditioned medium. HFSS patients are characterized by high levels of plasma IL-3, monocyte derived inhibitory activity(s) and circulating BFU-E are quiescent we hypothesize that the HFSS patients balance between inhibitory and stimulatory factors allow the proliferative response of stem cells to increase demand while lack of inhibitors and increased levels of stimulatory factors in LFSS patients maintain permanent state of stimulated hematopoiesis. Now we present data based on sTfR, Epo, SCF and TGF- plasma levels, which reinforces this model. Based on sTfR and Epo evaluation we demonstrated that the level of erythropoiesis is not identical in LFSS and HFSS patients, supporting the idea that different regulatory mechanisms are required to maintain the two levels of increase hematopoiesis. Red cell production was evaluated by the levels of soluble transferin receptors (sTfR) in the plasma of SS patients and controls. The sTfR concentration was increased in all SS samples as compare to controls (p <0.002) and sTfR levels are negatively correlated with the % of peripheral HbF. (r= -0.5744 p <0.002). Furthermore, the levels of sTfR were higher in LFSS than in HFSS patients. Epo levels were increased in the plasma of LFSS individuals (range = 34-215 ml U /ml), the values in HFSS patients were in normal range (3-20 ml U/ml ).We conclude that while erythropoiesis is generally increased in SS patients, LFSS patient exhibit particularly high levels of hematopoietic activity. We identify SCF and TGF-Beta as regulatory factors specifically affected by the presence of SS genotypes and its level of severity. The plasma concentrations of SCF and TGF-beta are modified in SS patients as compare to controls and tend to vary each other inversely with the levels of peripheral HbF. The highest levels of SCF (up to 7000 pg/ml) were detected in LFSS patients. These individuals also showed increased number of SCF+ CD34 enriched circulating cells (49%). Lower level of SCF factor in HFSS patients is associated with increased concentration of TGF- beta suggesting a regulatory role of TGF-beta on either SCF release or c-kit expression on progenitor cells. Data previously published complemented by the one presented here, supported a model in which HFSS patients achieve a balance between inhibitory (TGF-beta) and stimulatory (SCF, IL-3) factors, resulting in moderate erythropoietic response. In contrast, in LFSS patient, the low levels of TGF-beta and the increased release of GM-CSF and SCF maintain the intense erythropoiesis in response to erythropoietic stress in these more severe patients. These results give a base for quantitative and qualitative pattern of GF disregulation in SS anemia and are the accomplishments of Aim 2 and 3.